As one of the effective medical treatment methods to treat a patient for a disease of the respiratory such as pulmonary emphysema or chronic bronchitis, the medical treatment of oxygen inhalation is employed. In the medical treatment of oxygen inhalation, oxygen enriched gas, which is generated by separating nitrogen gas from air, is supplied to a patient. In order to accomplish this object, they have developed oxygen concentrators for generating oxygen enriched gas from air.
As an example of the oxygen concentrators described above, the U.S. Pat. No. 6,311,719 B1 describes a pressure swing adsorption (PSA) system provided with a plurality of nitrogen adsorption cylinders. For the medical treatment of oxygen inhalation conducted on a patient at home, a small oxygen concentrating apparatus, in which an oxygen concentrator and a compressor for supplying compressed air to the oxygen concentrator are combined with each other, has already been offered. Since this type oxygen concentrating apparatus is used at home, it is necessary to reduce the noise generated by the compressor and cooling fan.
In order to reduce the noise generated by each component of the oxygen concentrating apparatus, for example, the official gazettes of Japanese Unexamined Patent Publication Nos. 61-155204, 60-200804 and 2-211175 disclose an oxygen concentrating apparatus including: a sound insulation box made of metal surrounding a compressor and cooling fan; and an exhaust duct for guiding exhaust, which is sent from the cooling fan, to the outside, wherein the exhaust duct is formed into a bent passage having a plurality of bend sections.
However, the prior art described above has the following problems.
In the case where the sound insulation box made of metal surrounding the compressor and cooling fan is provided, when air is supplied from the compressor to the oxygen concentrator, pressure fluctuation is caused in the sound insulation box made of metal by the action of noise. Due to this pressure fluctuation, a side wall of the sound insulation box is vibrated especially in the low frequency region of 100 to 400 Hz. Therefore, the sound insulation box could become a new noise source.
When the plurality of bend sections are formed in the duct through which exhaust sent from the cooling fan is discharged, the exhaust noise generated from the cooling fan can be reduced, however, the entire size of the oxygen concentrating apparatus is extended, and the pressure loss of the duct is increased.